Magnetic field strength is measured by a number of methods with the nuclear magnetic resonance (NMR) gaussmeter being the most widely used device when high accuracy and precision are required. In the NMR device, nuclei with magnetic moments absorb radio-frequency energy (RF) at unique resonant frequencies proportional to the applied magnetic field. This is accomplished with a marginal oscillator coupled to a probe which is inserted in the magnetic field whose strength is to be measured. The probe comprises one-half of a resonant circuit enclosing the material containing the nuclei. The magnetic field changes the properties of the resonant circuit in combination with the nuclear magnetic properties of the nuclei. As a result, the NMR gaussmeter translates the problem of the magnetic field measurement to one of frequency measurement which can be precisely determined. That is, the frequency of the marginal oscillator in combination with the probe inserted in the magnetic field produces a resonance in a circuit having a frequency corresponding to the magnetic field strength.
One application for measuring magnetic field strength is superconducting magnets. It is desirable to calibrate and determine the magnetic field homogeneity of these magnets at liquid helium temperatures, that is, at cryogenic temperatures. The measurement of the magnetic field at cryogenic temperatures in a dewar with a moveable probe is difficult and inaccurate. The inaccuracies result from the limited physical size of the resonating coil and probe core in relationship to the volume of the superconducting magnet core whose magnetic field homogeneity is to be determined. In essence, the probe is many orders of magnitude greater in size than that essential to measure with high accuracy the various shifts in homogeneity of the superconducting magnet field. In addition, some probes that are commercially available use liquids which are not suitable for cryogenic temperatures.